1. Field of the Invention
The present invention relates to a method for manufacturing an optical disk with multiple layers of recording surfaces.
2. Description of Related Art
As shown in FIG. 1, a conventional optical disk 50 for a type of recording with multiple layers has a first disk 1 having a first and second information recording surface and a second disk 2 having a third and fourth information recording surface with both disks adhered to each other by using an adhesive 8 or the like. The first disk 1 has a configuration in which a first semi-transparent reflective film 4, an intermediate layer 5 having the second information recording surface, a second reflective film 6, and a protection film 7 are laminated in sequence on a first information recording surface of a first optically transparent substrate 3.
First pits 9 for carrying first information signals and second pits 10 for carrying second information signals are formed on the first information recording surface of the first optically transparent substrate 3 and the second information recording surface of the intermediate layer 5, respectively. These first and second pits 9 and 10 have such a shape as to be able to perform reading by means of a reproducing beam that is emitted from the side of the first optically transparent substrate 3 (in the direction shown by the arrow in the figure). The second disk 2 has the same configuration as that of the first disk 1 and an explanation is omitted.
The aforementioned optical disk 50 is manufactured in the processes as shown in FIG. 2A through FIG. 2E.
First, as shown in FIG. 2A, the first optically transparent substrate 3 is prepared on which the first semi-transparent reflective film 4 is formed on the first information recording surface on which the first pits 9 for carrying the first information signals are formed.
Subsequently, as shown in FIG. 2B, with the first reflective layer 4 directed upward, the aforementioned first optically transparent substrate 3 is placed on a spin table 20. The spin table 20 is formed of a disk having a high degree of optical transparency and is provided with a shaft 21, on the center thereof, for attaching the substrate. Since there is provided a center hole 3a at the center of the first optically transparent substrate 3, the first optically transparent substrate 3 is fixedly absorbed to the spin table 20 by inserting the center hole 3a of the first optically transparent substrate 3 onto the shaft 21 of the spin table 20.
Subsequently, a resin P that hardens under an ultraviolet light and which becomes the intermediate layer 5, in a process to be described later, is deposited on the first reflective layer 4 by means of a dispenser (not shown) in the shape of a donut around the center hole 3a of the first optically transparent substrate 3. Thereafter, a stamper 22, on which the second pits 10 for carrying the second information signals are formed, is placed over the first reflective film 4 with the surface where the second pits 10 are formed directed downwardly. Since there is formed a center hole 22a at the center of the stamper 22 in the same way as the center hole 3a of the aforementioned first optically transparent substrate 3, the stamper 22 is placed on the same shaft as the first optically transparent substrate 3 by inserting the center hole 22a of the stamper 22 onto the shaft 21 of the spin table 20.
Subsequently, as shown in FIG. 2C, high-speed rotation of the spin table 20 causes the resin P to spread into the spacing between the stamper 22 and the first reflective film 4, and causes the excessive amount of resin P to be thrown off to form a film of the resin P, uniform in thickness. Subsequently, in the process shown in FIG. 2D, the intermediate layer 5 is formed by irradiating the bottom side of the spin table 20 (in the direction shown by the arrow in the figure) with ultraviolet light so as to harden the resin P.
Subsequently, as shown in FIG. 2E, the stamper 22 is removed to reveal the second pits 10 for carrying the second information signals transferred on the intermediate layer 5. The second reflective layer 6 having a reflectance higher than that of the first reflective film 4 is formed on the second information recording surface to which the second pits of the intermediate layer 5 are transferred. Thereafter, the protection film 7 is formed on the second reflective film 6 to complete the first disk 1.
Then, the second disk 2 with third and fourth information recording surfaces formed by the same processes as those described above and the first disk 1 manufactured in the aforementioned processes are adhered to each other by means of the adhesive 8 with the respective protection films 7 facing each other to complete the optical disk 50.
The optical disk 50 manufactured as described above is designed, for example, so as to read information signals provided by the first pits 9 with the light derived from the semi-transparent reflective film and reflected from the first reflective film 4, the semi-transparent reflective film allows a laser beam incident via the first optically transparent substrate 3 to transmit partially and reflect partially. The optical disk 50 is also designed so as to read information signals provided by the second pits 10 with the light which is transmitted through the first reflective film 4 and the intermediate layer 5, and then reflected on the second reflective film 6. Therefore, it is very important in providing improved reliability in reading information signals to make the intermediate layer 5 uniform in thickness.
It is difficult to make the thickness of the intermediate layer uniform since the aforementioned optical disk 50 employs the liquid-state ultraviolet light hardening resin P in the intermediate layer 5 due to the easy formation of air bubbles therein.
Accordingly, in the case of carrying out the rotational processing by means of the spin table 20, the processing is carried out in a chamber (not shown) in a reduced-pressure state to remove air bubbles contained in the intermediate layer 5. In addition, the intermediate layer 5 is made uniform in thickness by varying the amount of drops of the resin P, and the speed and time of the spin table 20. However, it was very difficult to make the intermediate layer 5 uniform in thickness by controlling the amount of drops of the resin P, and the speed and time of the spin table 20. Because of this problem, air bubbles were not able to be completely removed in the chamber having a reduced pressure.